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Title

INVESTIGATION THE EFFECT OF CABINET DRYING CONDITIONS AND DIFFERENT PRETREATMENTS ON DRYING RATE OF GRAPE AND QUALITY OF RAISIN

Pages

 Start Page 226 | End Page 238

Abstract

 Introduction: RAISIN is ripped and dried form of grape. DRYING is one of the most important steps in RAISIN processing. Achieving optimum DRYING conditions can affect the processing time and improvement of RAISIN quality. Temperature, AIR VELOCITY and pretreatment are important factors in grape DRYING process and its quality. One of the most important physical changes that the food suffers during DRYING is the reduction of its external volume. Loss of water and heating cause stresses in the cellular structure of the food leading to change in shape and decrease in dimension. Water movement and other solute materials influence the texture characteristics. Changes in shape, loss of volume and increased HARDNESS cause in most cases a negative impression in the consumer. There are, on the other hand, some dried products that have had traditionally a shrunken aspect, a requirement for the consumer of RAISINs or dried apricots. The purpose of this research was study effect of DRYING conditions (drying air temperature, AIR VELOCITY and pretreatments) on DRYING grape process and quality of RAISINMaterials and method: Materials: Grape (cv Askari) prepared from Kashmar gardens. Harvested samples have been kept at 4Ċ. After measuring moisture, diameter and Brix, the safe seeds are separated from their cluster. In order to pretreatment process grapes are dipped into the following solutions:-aqueous solution of potassium carbonate (2.5% w/w), and 2% Paksan oil-aqueous solution of 2% olive oil-hot water-distilled water (control)Experiments were performed at air temperatures of 60, 70 and 80Ċ and air velocities of 1.5 and 2.5 m. s-1. A batch cabinet dryer containing three perforated trays was used for DRYING the samples. The thermal condition was variable and the operational temperature could be tuned with a built-in thermocouple. The grapes were put on the tray in single layer after pre-treatment. DRYING process continued to reduce the moisture content of the grape samples to 13% (wet basis). After DRYING process the RAISIN samples were packaged in a 3 laminated layer bags (PE/PA/PE) and kept at temperature 4Ċ. The dried sample was analyzed for its quality by estimating the rehydration, SHRINKAGE, HARDNESS and browning index. To calculate the DRYING rate at any time of the DRYING process, the sample weight at the time and the next time were measured. SHRINKAGE was determined from the change in volume of RAISIN prior and after DRYING. Rehydration properties were determined by immersing dried samples in boiling water (100Ċ) for 10 min. Browning index was measured in according to the method developed by Baloch et al. Extraction of the water soluble brown pigment was carried out with acetic acid containing formaldehyde. Absorbance of supernatants was recorded at 420 and 600 nm, using an UV-VIS double-beam spectrophotometer (S2000 uv/vis light wave). The browning index was calculated by subtracting absorbance at 600 nm (for turbidity) from that of 420 nm. HARDNESS of RAISINs was determined in a puncture testusing a QTS texture analyzer (CNS Farnelll, Essex, UK) equipped with a needle probe (stainless steel cylinder of 2 mm of diameter with a conical needle bit) and a test speed of 60 mm. min-1 during the test. HARDNESS was defined as maximum force to puncture RAISIN from the top to a 2mm depth. Analysis of variance (ANOVA) was conducted for data using MSTAT-C software. Differences among the mean values were also determined using Duncan’s Multiple Range test. A significant level was defined as a probability of 0.05.Results and Discussion: The results showed that temperature, AIR VELOCITY and pretreatment have significant effects on DRYING rate and quality of RAISIN. DRYING rate increased with increasing temperature, AIR VELOCITY and with application of preatreatment. The waxy cuticle of grape skin controls the rate of moisture diffusion through the berries and, in order to accelerate DRYING, chemical treatments are applied to remove or modify this cuticle and increase grape skin permeability to water. The results showed that preatreatment of potassium carbonate and Paksan oil had most effect in accelerating DRYING process. Rehydration depends on structural changes in vegetal tissues and cells of food material during DRYING, which produces SHRINKAGE, collapse and reduces the water absorption capacity, thereby preventing the complete rehydration of the dried product. The pretreated grapes showed increased rehydration. Increasing temperature and application of preatreatment caused a reduction in SHRINKAGE. It was found that increasing DRYING temperature increased the HARDNESS of RAISIN and treating the grape caused reducing the HARDNESS of RAISIN. Increasing DRYING temperature resulted in higher browning index. The treated grape by potassium carbonate and Paksan oil had least browning index. Increasing AIR VELOCITY decreased browning index. The best values of HARDNESS (0.73 N), SHRINKAGE (81/04%) and the least value of browning index (0/157) were related to dried RAISIN at 60Ċ and most value of rehydration (1/266) was related to dried RAISIN at 70Ċ. AIR VELOCITY just significantly affected on browning index. Also between used preatreatments, potassium carbonate and paksan oil caused best physicochemical results.

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